Implanted leads for stimulation and sensing of organs are used for many years in various anatomical locations. Typical applications are in cardiac pacemakers and more recently spinal pain relief and control of gastro-internal symptoms and processes.
In the latter context, leads are implanted in the abdomen and attached, for example, to the stomach wall where they are subjected to conditions of tension and varying bending over a considerable distance (˜6 cm) while conducting current from a pulse generator. These conditions may contribute to a failure, or fatigue, of the lead due to mechanical stresses and, possibly, corrosion.
As with other implantable devices, the lead's safety and reliability are highly important. An approach to verify a lead's safety and reliability is to subject it to conditions that simulate, or exceed, its operating conditions in the body and to ascertain its functional compliance to specified criteria.
Such an approach is found in BS EN 45501-2-1. This is an established standard for testing pacemaker leads, where various characteristics of their operation is simulated and tested, typically one, or two at a time.
As to mechanical fatigue, the standard specifies bending and un-bending a dry lead at room temperature around a curved corner. One test defines bending in a range of ±90° with bend radius of ˜6 mm where a section of ˜9 mm, with minimal tension, undergoes 47,000 bending cycles at a rate of 2 Hz. In order to test the relevant part of the lead, the procedure may have to be repeated separately for each of its unique flexible parts.
Another test, for the fatigue of the interface between the lead and its connector, specifies bending in a range of ±45° with a bend radius of ˜0.5 mm, where a section of less than 1 mm undergoes 82,000 bending cycles at a rate of 2 Hz, with an applied tension of 100 gr.
Thus, even according to the first test procedure that tests the lead's body fatigue, the bending of a single segment of 9 mm length requires over 6.5 hours.
Another procedure is described in patent GB 878 401. This reference teaches wrapping a wire around two semi-circular ‘rockers’, which bend and un-bends the wire in alternate directions as the rockers oscillate against each other in reciprocal orientations.
In both of the last two references, the bending section length and bend curvature are mutually related; that is, the tighter the bend the smaller the bent section, and vise versa, the larger the section the shallower the bend.